208996 Achlys

Discovery

Achlys was discovered on 13 January 2003 by astronomers Chad Trujillo and Michael Brown^[d] at Palomar Observatory in San Diego County, California.^[1] At the time, Trujillo and Brown were searching the sky for bright and large trans-Neptunian objects—Solar System objects beyond Neptune—using the Palomar Observatory's 1.22-meter (48 in) Samuel Oschin telescope, as part of their "Caltech Wide Area Sky Survey" project.^{[14]: 100, 103 } Their sky survey had been operating jointly with Palomar's Near Earth Asteroid Tracking (NEAT) program since 2001 and was responsible for the discovery of several other large trans-Neptunian objects including Quaoar.^{[14]: 99–100 }

Achlys was found through manual vetting of potential moving objects identified by Trujillo and Brown's automated image-searching software.^{[14]: 101 } It was detected at a red-filter apparent magnitude of 20.2.^[15] The discovery of Achlys was announced by Minor Planet Center (MPC) on 26 January 2003.^[15] Since then, numerous precovery observations of Achlys were identified from Palomar Observatory, with observations from 2001–2002 reported in April 2003^[16] and a single observation from 19 March 1996 reported in December 2007.^[17] This 1996 observation is the earliest known precovery of Achlys, predating its discovery by over six years.^[1]

The 1.2-meter Samuel Oschin telescope that was used to discover Achlys at Palomar Observatory

Discovery images of Achlys from 13 January 2003

Name

This object is named after Achlys, the goddess of sorrow and grief in the ancient Greek epic poem Shield of Heracles. In Homer's Iliad, "achlys" also refers to the mist that covers the eyes of the dying.^{[18]: 16 } This name follows the theme of mythological figures related to the underworld, which is required for plutinos by the International Astronomical Union (IAU).^{[19]: 8 } The object was officially named Achlys on 30 June 2025 by the IAU's Working Group for Small Body Nomenclature.^{[18]: 24 }

Before Achlys was named, it was known by its provisional designation 2003 AZ₈₄, which was given by the MPC in its discovery announcement.^[15] This provisional designation encodes the year and half-month of Achlys's discovery date.^[20] Achlys's minor planet catalog number of 208996 was given by the MPC on 11 March 2009.^[21]

Occultations

While moving across the sky, Achlys occasionally passes in front of a background star and briefly blocks out its light from Earth, resulting in a stellar occultation. When observed at different locations, stellar occultations by Achlys can reveal fine details that are unresolvable to telescope imaging, such as its size, shape, and potential surrounding features such as rings and moons.^{[22]: 2 [8]: 2 } The first successful detection of a stellar occultation by Achlys was reported by a single observer from Chile on 8 January 2011.^{[23][8]: 2 } Later observations of stellar occultations on 3 February 2012 and 15 November 2014 were successfully detected by multiple observers at different locations, which revealed the elongated shape and topography of Achlys for the first time.^{[8]: 2 } No rings and moons were detected around Achlys in these observations.^{[8]: 8 }

Size, shape, density

Observations of stellar occultations from 2012 and 2014 show that Achlys is an elongated object.^[8] Under the assumption that Achlys is in hydrostatic equilibrium (that is, its shape is controlled by its own gravity and rotation), its shape can be approximated by a rotationally distorted Jacobi ellipsoid with dimensions of 940 km × 766 km × 490 km (584 mi × 476 mi × 304 mi).^{[8]: 1 } With these dimensions, the equatorial diameter of Achlys is roughly twice as long as its polar diameter.^{[32]: 3 } This rotationally distorted shape has been seen in other large Kuiper belt objects, like Haumea and Varuna.^{[8]: 13 } The approximated dimensions of Achlys translate to an area-equivalent diameter of 772 km (480 mi),^{[8]: 10 } which is large enough that astronomers consider Achlys likely in hydrostatic equilibrium^{[f][9]: 10 } and therefore a possible dwarf planet.^{[33]: 245 [34]: 7, 13 } Achlys is among the top 30 largest known TNOs.^{[31]: 2 }

The mass and density of Achlys have not been measured, though the assumption of hydrostatic equilibrium with its ellipsoidal shape predicts that it should have a density in the range of 0.85–1.12 g/cm³, with the most likely value being 0.87 g/cm³.^{[8]: 9–10 } This density indicates Achlys has a mass of around 2×10²⁰ kg.^[c] Achlys falls within the 400–1,000 km (250–620 mi) diameter range where TNOs are typically observed with densities lower than that of water ice (1 g/cm³); these objects are theorized to have porous interior structures due to a lack of internal melting, differentiation, and gravitational compression.^{[35]: 1, 8 [36]: 5 } Achlys's small size also makes it unlikely to hold much internal heat or possess cryovolcanism.^{[37]: 10 }

Rotation

Achlys has a rotation period of about 6.79 hours.^{[9]: 10 } The apparent brightness of Achlys periodically fluctuates with a small peak-to-peak amplitude of 0.07 magnitudes as it rotates, which could be monitored by telescopes on Earth.^{[9]: 10 } The rotation period of Achlys was first measured by Scott Sheppard and David C. Jewitt in 2003, who found a rotation period of either 6.7 and 13.4 hours.^{[38]: 217 [39]: 7 } The rotation period of Achlys remained ambiguous until 2017, when it was found to have an elongated shape that was best explained by the former period.^{[8]: 9 }

Achlys has a highly tilted rotation axis that is pointed somewhat toward Earth, according to observations of Achlys's small rotational brightness variation, constant thermal emission, and variable projected shape in stellar occultations.^{[8]: 9–10 [9]: 11 } A 2017 analysis of stellar occultation observations from 2012 and 2014 suggested that Achlys's rotational pole could be oriented at an opening angle of roughly 50° with respect to the plane of the sky and at a position angle of 78° eastward from the celestial north pole.^{[8]: 9–10 }

Surface

Achlys is a dark, icy object with a likely ancient surface devoid of geological activity.^{[41]: 1507 } Its surface is dominated by water ice and has a low, average geometric albedo of about 10%.^{[9]: 10 } Astronomical spectroscopy has shown that both amorphous and crystalline forms of water ice are present on Achlys's surface, which make it highly absorbent in near-infrared wavelengths of light.^{[42][40][37]: 9 } Crystalline water ice is not commonly found on the surfaces of TNOs due to their cold temperatures, so its presence on Achlys suggests that the object had experienced heating, possibly by an impact event.^{[43]: 946, 953 }

Spectroscopic analyses from 2010 and 2011 suggest that Achlys's surface may be darkened by amorphous carbon, which is thought to originate from heavily irradiated organic compounds.^{[43]: 953 } These analyses also suggest that water ice and amorphous carbon each comprise major portions of Achlys's surface, with amorphous water ice likely more abundant than crystalline water ice.^{[42]: 301 [43]: 949 } The surface of Achlys is devoid of volatile ices like nitrogen and carbon monoxide, in contrast to the largest trans-Neptunian objects like Pluto.^{[37]: 9 } This is expected because Achlys's gravity is too weak to hold on any atmosphere, especially volatiles after they have sublimated into vapor.^{[41]: 1502, 1507 }

In visible light, Achlys's surface is spectrally neutral with respect to the Sun,^[g] which gives it a gray color.^{[44][11]: 181, 188 } Achlys shares its gray color and water ice-rich surface with several TNOs including the large plutino Orcus;^[45] these objects are classified as BB ("blue")-type TNOs in terms of visible color^[11] and "prominent water" (H
₂O)-type TNOs in terms of spectra.^{[10]: 6 } The gray color of Achlys implies that it contains little amounts of tholins—moderately irradiated organic compounds that would otherwise tint the surface red.^{[43]: 949, 953 } Spectroscopic analyses suggest that tholins should make up roughly 10% of Achlys's surface composition.^{[42]: 301 [43]: 949 }

The surface of Achlys is not uniform.^{[37]: 9 } The albedo is suspected to vary across Achlys's surface because the object varies in brightness as it rotates.^{[8]: 10 } Near-infrared spectroscopy of Achlys shows variable spectral absorption of water ice and tentative signs of methanol ice.^{[37]: 9 } Visible-light spectroscopy has detected sporadic hints of hydrated minerals such as silicates, which are suspected to exist in localized deposits on Achlys's surface.^{[46]: 464 [37]: 9 } The color of Achlys has been observed to change as it rotates, with its visible spectral slope or redness varying between 3.5%/0.1 Î¼m and 8.5%/0.1 Î¼m.^{[37]: 9 } This color variation may hint at localized areas of concentrated tholins on Achlys's surface, which could be explained by various phenomena such as an impact crater left by a red TNO.^{[37]: 9 }

In November 2014, an observation of a stellar occultation from Yunnan, China revealed that Achlys has a topographic feature located at its limb—the edge of its projected shape.^{[8]: 1–2 } The observation showed a gradual dimming of the occulted star, which has been interpreted as a partial, grazing occultation by Achlys's topographic feature.^{[8]: 10 [32]: 24 } Achlys is the first TNO whose topographic feature has been observed via stellar occultation.^{[47][8]: 2, 13 } The topographic feature on Achlys could either be a chasm at least 7.7 km (4.8 mi) deep and roughly 22.6 km (14.0 mi) wide, or a shallow-sloped depression roughly 13.4 km (8.3 mi) deep and at least 80 km (50 mi) wide.^{[8]: 12–13 } Such features have been seen on Pluto and its moon Charon.^{[48][8]: 12 }